gen_event

Generic Event Handling Behaviour

A behaviour module for implementing event handling functionality.
The OTP event handling model consists of a generic event manager
process with an arbitrary number of event handlers which are added and
deleted dynamically.

An event manager implemented using this module will have a standard
set of interface functions and include functionality for tracing and
error reporting. It will also fit into an OTP supervision tree.
Refer to OTP Design Principles for more information.

Each event handler is implemented as a callback module exporting
a pre-defined set of functions. The relationship between the behaviour
functions and the callback functions can be illustrated as follows:

Since each event handler is one callback module, an event manager
will have several callback modules which are added and deleted
dynamically. Therefore gen_event is more tolerant of callback
module errors than the other behaviours. If a callback function for
an installed event handler fails with Reason, or returns a
bad value Term, the event manager will not fail. It will delete
the event handler by calling the callback function
Module:terminate/2 (see below), giving as argument
{error,{'EXIT',Reason}} or {error,Term}, respectively.
No other event handler will be affected.

A gen_event process handles system messages as documented in
sys(3). The sys module
can be used for debugging an event manager.

Note that an event manager does trap exit signals
automatically.

The gen_event process can go into hibernation
(see erlang(3)) if a callback
function in a handler module specifies 'hibernate' in its return value.
This might be useful if the server is expected to be idle for a long
time. However this feature should be used with care as hibernation
implies at least two garbage collections (when hibernating and
shortly after waking up) and is not something you'd want to do
between each event handled by a busy event manager.

It's also worth noting that when multiple event handlers are
invoked, it's sufficient that one single event handler returns a
'hibernate' request for the whole event manager to go into
hibernation.

Unless otherwise stated, all functions in this module fail if
the specified event manager does not exist or if bad arguments are
given.

start_link(EventMgrName) -> Result

Creates an event manager process as part of a supervision
tree. The function should be called, directly or indirectly,
by the supervisor. It will, among other things, ensure that
the event manager is linked to the supervisor.

If EventMgrName={local,Name}, the event manager is
registered locally as Name using register/2.
If EventMgrName={global,GlobalName}, the event manager is
registered globally as GlobalName using
global:register_name/2. If no name is provided,
the event manager is not registered.
If EventMgrName={via,Module,ViaName}, the event manager will
register with the registry represented by Module.
The Module callback should export the functions
register_name/2, unregister_name/1,
whereis_name/1 and send/2, which should behave like the
corresponding functions in global. Thus,
{via,global,GlobalName} is a valid reference.

If the event manager is successfully created the function
returns {ok,Pid}, where Pid is the pid of
the event manager. If there already exists a process with
the specified EventMgrName the function returns
{error,{already_started,Pid}}, where Pid is
the pid of that process.

Adds a new event handler to the event manager EventMgrRef.
The event manager will call Module:init/1 to initiate
the event handler and its internal state.

EventMgrRef can be:

the pid,Name, if the event manager is locally registered,{Name,Node}, if the event manager is locally
registered at another node, or{global,GlobalName}, if the event manager is globally
registered.{via,Module,ViaName}, if the event manager is registered
through an alternative process registry.

Handler is the name of the callback module Module or
a tuple {Module,Id}, where Id is any term.
The {Module,Id} representation makes it possible to
identify a specific event handler when there are several event
handlers using the same callback module.

Args is an arbitrary term which is passed as the argument
to Module:init/1.

If Module:init/1 returns a correct value indicating
successful completion, the event manager adds the event
handler and this function returns
ok. If Module:init/1 fails with Reason or
returns {error,Reason}, the event handler is
ignored and this function returns {'EXIT',Reason} or
{error,Reason}, respectively.

Adds a new event handler in the same way as add_handler/3
but will also supervise the connection between the event handler
and the calling process.

If the calling process later terminates with Reason,
the event manager will delete the event handler by calling
Module:terminate/2 with {stop,Reason} as argument.

If the event handler later is deleted, the event manager
sends a message{gen_event_EXIT,Handler,Reason} to
the calling process. Reason is one of the following:

normal, if the event handler has been removed due to a
call to delete_handler/3, or remove_handler
has been returned by a callback function (see below).shutdown, if the event handler has been removed
because the event manager is terminating.{swapped,NewHandler,Pid}, if the process Pid
has replaced the event handler with another event handler
NewHandler using a call to swap_handler/3 or
swap_sup_handler/3.a term, if the event handler is removed due to an error.
Which term depends on the error.

See add_handler/3 for a description of the arguments
and return values.

notify(EventMgrRef, Event) -> ok

sync_notify(EventMgrRef, Event) -> ok

Sends an event notification to the event manager
EventMgrRef. The event manager will call
Module:handle_event/2 for each installed event handler to
handle the event.

notify is asynchronous and will return immediately after
the event notification has been sent. sync_notify is
synchronous in the sense that it will return ok after
the event has been handled by all event handlers.

See add_handler/3 for a description of EventMgrRef.

Event is an arbitrary term which is passed as one of
the arguments to Module:handle_event/2.

notify will not fail even if the specified event manager
does not exist, unless it is specified as Name.

Makes a synchronous call to the event handler Handler
installed in the event manager EventMgrRef by sending a
request and waiting until a reply arrives or a timeout occurs.
The event manager will call Module:handle_call/2 to handle
the request.

See add_handler/3 for a description of EventMgrRef
and Handler.

Request is an arbitrary term which is passed as one of
the arguments to Module:handle_call/2.

Timeout is an integer greater than zero which specifies
how many milliseconds to wait for a reply, or the atom
infinity to wait indefinitely. Default value is 5000.
If no reply is received within the specified time, the function
call fails.

The return value Reply is defined in the return value of
Module:handle_call/2. If the specified event handler is not
installed, the function returns {error,bad_module}. If
the callback function fails with Reason or returns an
unexpected value Term, this function returns
{error,{'EXIT',Reason}} or {error,Term},
respectively.

Deletes an event handler from the event manager
EventMgrRef. The event manager will call
Module:terminate/2 to terminate the event handler.

See add_handler/3 for a description of EventMgrRef
and Handler.

Args is an arbitrary term which is passed as one of
the arguments to Module:terminate/2.

The return value is the return value of Module:terminate/2.
If the specified event handler is not installed, the function
returns {error,module_not_found}. If the callback function
fails with Reason, the function returns
{'EXIT',Reason}.

Replaces an old event handler with a new event handler in
the event manager EventMgrRef.

See add_handler/3 for a description of the arguments.

First the old event handler Handler1 is deleted.
The event manager calls Module1:terminate(Args1, ...),
where Module1 is the callback module of Handler1,
and collects the return value.

Then the new event handler Handler2 is added and initiated
by calling Module2:init({Args2,Term}), where Module2
is the callback module of Handler2 and Term
the return value of Module1:terminate/2. This makes it
possible to transfer information from Handler1 to
Handler2.

The new handler will be added even if the the specified old event
handler is not installed in which case Term=error, or if
Module1:terminate/2 fails with Reason in which case
Term={'EXIT',Reason}.
The old handler will be deleted even if Module2:init/1
fails.

If there was a supervised connection between Handler1 and
a process Pid, there will be a supervised connection
between Handler2 and Pid instead.

If Module2:init/1 returns a correct value, this function
returns ok. If Module2:init/1 fails with
Reason or returns an unexpected value Term, this
this function returns {error,{'EXIT',Reason}} or
{error,Term}, respectively.

Whenever a new event handler is added to an event manager,
this function is called to initialize the event handler.

If the event handler is added due to a call to
gen_event:add_handler/3 or
gen_event:add_sup_handler/3, InitArgs is
the Args argument of these functions.

If the event handler is replacing another event handler due to
a call to gen_event:swap_handler/3 or
gen_event:swap_sup_handler/3, or due to a swap
return tuple from one of the other callback functions,
InitArgs is a tuple {Args,Term} where Args is
the argument provided in the function call/return tuple and
Term is the result of terminating the old event handler,
see gen_event:swap_handler/3.

If successful, the function should return {ok,State}
or {ok,State,hibernate} where State is the
initial internal state of the event handler.

If {ok,State,hibernate} is returned, the event
manager will go into hibernation (by calling proc_lib:hibernate/3),
waiting for the next event to occur.

Module:handle_event(Event, State) -> Result

Event = term()

State = term()

Result = {ok,NewState} | {ok,NewState,hibernate}

| {swap_handler,Args1,NewState,Handler2,Args2} | remove_handler

NewState = term()

Args1 = Args2 = term()

Handler2 = Module2 | {Module2,Id}

Module2 = atom()

Id = term()

Whenever an event manager receives an event sent using
gen_event:notify/2 or gen_event:sync_notify/2, this
function is called for each installed event handler to handle
the event.

Event is the Event argument of
notify/sync_notify.

State is the internal state of the event handler.

If the function returns {ok,NewState} or {ok,NewState,hibernate}
the event handler
will remain in the event manager with the possible updated
internal state NewState.

If {ok,NewState,hibernate} is returned, the event
manager will also go into hibernation (by calling proc_lib:hibernate/3),
waiting for the next event to occur. It is sufficient that one of the event
handlers return {ok,NewState,hibernate} for the whole event manager
process to hibernate.

If the function returns
{swap_handler,Args1,NewState,Handler2,Args2} the event
handler will be replaced by Handler2 by first calling
Module:terminate(Args1,NewState) and then
Module2:init({Args2,Term}) where Term is the return
value of Module:terminate/2.
See gen_event:swap_handler/3 for more information.

If the function returns remove_handler the event handler
will be deleted by calling
Module:terminate(remove_handler,State).

Module:handle_call(Request, State) -> Result

Request = term()

State = term()

Result = {ok,Reply,NewState} | {ok,Reply,NewState,hibernate}

| {swap_handler,Reply,Args1,NewState,Handler2,Args2}

| {remove_handler, Reply}

Reply = term()

NewState = term()

Args1 = Args2 = term()

Handler2 = Module2 | {Module2,Id}

Module2 = atom()

Id = term()

Whenever an event manager receives a request sent using
gen_event:call/3,4, this function is called for
the specified event handler to handle the request.

Request is the Request argument of call.

State is the internal state of the event handler.

The return values are the same as for handle_event/2
except they also contain a term Reply which is the reply
given back to the client as the return value of call.

Module:handle_info(Info, State) -> Result

Info = term()

State = term()

Result = {ok,NewState} | {ok,NewState,hibernate}

| {swap_handler,Args1,NewState,Handler2,Args2} | remove_handler

NewState = term()

Args1 = Args2 = term()

Handler2 = Module2 | {Module2,Id}

Module2 = atom()

Id = term()

This function is called for each installed event handler when
an event manager receives any other message than an event or
a synchronous request (or a system message).

Info is the received message.

See Module:handle_event/2 for a description of State
and possible return values.

Module:terminate(Arg, State) -> term()

Arg = Args | {stop,Reason} | stop | remove_handler

| {error,{'EXIT',Reason}} | {error,Term}

Args = Reason = Term = term()

Whenever an event handler is deleted from an event manager,
this function is called. It should be the opposite of
Module:init/1 and do any necessary cleaning up.

If the event handler is deleted due to a call to
gen_event:delete_handler, gen_event:swap_handler/3
or gen_event:swap_sup_handler/3, Arg is
the Args argument of this function call.

Arg={stop,Reason} if the event handler has a supervised
connection to a process which has terminated with reason
Reason.

Arg=stop if the event handler is deleted because
the event manager is terminating.

The event manager will terminate if it is part of a supervision
tree and it is ordered by its supervisor to terminate.
Even if it is not part of a supervision tree, it will
terminate if it receives an 'EXIT' message from
its parent.

Arg=remove_handler if the event handler is deleted because
another callback function has returned remove_handler or
{remove_handler,Reply}.

Arg={error,Term} if the event handler is deleted because
a callback function returned an unexpected value Term,
or Arg={error,{'EXIT',Reason}} if a callback function
failed.

State is the internal state of the event handler.

The function may return any term. If the event handler is
deleted due to a call to gen_event:delete_handler,
the return value of that function will be the return value of this
function. If the event handler is to be replaced with another event
handler due to a swap, the return value will be passed to
the init function of the new event handler. Otherwise
the return value is ignored.

Module:code_change(OldVsn, State, Extra) -> {ok, NewState}

OldVsn = Vsn | {down, Vsn}

Vsn = term()

State = NewState = term()

Extra = term()

This function is called for an installed event handler which
should update its internal state during a release
upgrade/downgrade, i.e. when the instruction
{update,Module,Change,...} where
Change={advanced,Extra} is given in the .appup
file. See OTP Design Principles for more
information.

In the case of an upgrade, OldVsn is Vsn, and
in the case of a downgrade, OldVsn is
{down,Vsn}. Vsn is defined by the vsn
attribute(s) of the old version of the callback module
Module. If no such attribute is defined, the version
is the checksum of the BEAM file.

State is the internal state of the event handler.

Extra is passed as-is from the {advanced,Extra}
part of the update instruction.

The function should return the updated internal state.

Module:format_status(Opt, [PDict, State]) -> Status

Opt = normal | terminate

PDict = [{Key, Value}]

State = term()

Status = term()

Note!

This callback is optional, so event handler modules need
not export it. If a handler does not export this function,
the gen_event module uses the handler state directly for
the purposes described below.

This function is called by a gen_event process when:

One
of sys:get_status/1,2
is invoked to get the gen_event status. Opt is set
to the atom normal for this case.The event handler terminates abnormally and gen_event
logs an error. Opt is set to the
atom terminate for this case.

This function is useful for customising the form and
appearance of the event handler state for these cases. An
event handler callback module wishing to customise
the sys:get_status/1,2 return value as well as how
its state appears in termination error logs exports an
instance of format_status/2 that returns a term
describing the current state of the event handler.

PDict is the current value of the gen_event's
process dictionary.

State is the internal state of the event
handler.

The function should return Status, a term that
customises the details of the current state of the event
handler. Any term is allowed for Status. The
gen_event module uses Status as follows:

When sys:get_status/1,2 is called, gen_event
ensures that its return value contains Status in
place of the event handler's actual state term.When an event handler terminates abnormally, gen_event
logs Status in place of the event handler's actual
state term.

One use for this function is to return compact alternative
state representations to avoid having large state terms
printed in logfiles.